(1) Field of the Invention
This invention relates to a thermal energy scavenger or a thermal energy converting assembly of the type for converting heat energy into mechanical energy and, more specifically, to such an assembly utilizing a plurality of temperature-sensitive elements made of a material which exhibits shape memory due to a thermoelastic, martensitic phase transformation whereby less energy is required to strain the elements in a cold condition than the energy returned when the elements become unstrained as they are heated to a higher temperature.
During recent years various materials comprising metal alloys have been developed which have a shape memory characteristic based upon thermoplastic, martensitic phase transformations which are stress or strain dependent. Basically, such alloys exhibit a stable shape in a phase above a given transition temperature and experience a transformation to a martensitic phase at a temperature below the transition temperature. The alloys have a much lower effective modulus at the martensitic phase below the transition temperature thereby requiring a relatively small amount of energy in the form of stress for straining the alloy when at the lower temperature whereas the alloy provides much more energy as it unstrains and returns to its original shape when it reaches a higher temperature above the transition temperature. Examples of alloys which have this shape memory characteristic are titanium-nickel; copper-aluminum-nickel; copper-zinc; iron-platinum and gold-cadmium.
A discussion of the shape memory characteristics in a number of alloys is set forth in the Journal of Material Science; 1974, Volume 9, Pages 15-21 by the authors L. Delaey, R. V. Krishnan and H. Tas. Further discussions are set forth in Metallurgical Transactions; 1975, Volume 6A, Page 29 by H. C. Tong and C. M. Wayman.
Further description of materials having the shape memory characteristic are set forth in U.S. Pat. No. 3,174,851 granted Mar. 23, 1965 to William J. Buehler and Raymond C. Wiley and U.S. Pat. No. 3,558,369 granted to F. E. Wang and William J. Buehler on Jan. 26, 1971.
(2) Description of the Prior Art
There have been efforts to utilize these materials, which have shape memory characteristics, in thermal energy converting assemblies and such assemblies have proved that the materials may be so utilized. Such assemblies strain the material having the shape memory characteristic and extract energy but have not efficiently utilized the material in the manner in which the material is strained nor maximized the amount of material strained in a given space. Additionally, the material having the shape memory characteristic may be strained to a greater extent while below the transition temperature than it may be strained while above the transition temperature and the prior assemblies may strain the material equally when both below and above the transition temperature.